network: response stacking for all commands

This change refactors most of memcached's networking frontend code. The
goals with this change:

- Decouple buffer memory from client connections where plausible,
reducing memory usage for currently inactive TCP sessions.
- Allow easy response stacking for all protocols. Previously every
binary command generates a syscall during response. This is no longer
true when multiple commands are read off the wire at once. The new meta
protocol and most text protocol commands are similar.
- Reduce code complexity for network handling. Remove automatic buffer
adjustments, error checking, and so on.

This is accomplished by removing the iovec, msg, and "write buffer"
structures from connection objects. A `mc_resp` object is now used to
represent an individual response. These objects contain small iovec's
and enough detail to be late-combined on transmit. As a side effect,
UDP mode now works with extstore :)

Adding to the iovec always had a remote chance of memory failure, so
every call had to be checked for an error. Now once a response object
is allocated, most manipulations can happen without any checking. This
is both a boost to code robustness and performance for some hot paths.

This patch is the first in a series, focusing on the client response.

1 files changed, 3 insertions, 0 deletions

diff --git a/doc/protocol.txt b/doc/protocol.txt
index abe70b2..85b92f5 100644
--- a/doc/protocol.txt
+++ b/doc/protocol.txt
@@ -1097,6 +1097,9 @@ integers separated by a colon (treat this as a floating point number).
 | rejected_connections  | 64u     | Conns rejected in maxconns_fast mode      |
 | connection_structures | 32u     | Number of connection structures allocated |
 |                       |         | by the server                             |
+| response_obj_bytes    | 64u     | Number of bytes used for response objects |
+| response_obj_total    | 64u     | Total nummber of response objects         |
+| response_obj_free     | 64u     | Current free cached response objects      |
 | reserved_fds          | 32u     | Number of misc fds used internally        |
 | cmd_get               | 64u     | Cumulative number of retrieval reqs       |
 | cmd_set               | 64u     | Cumulative number of storage reqs         |